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The Everchanging Pulsating White Dwarf GD358 Astronomy & Astrophysics manuscript no. gd358 January 23, 2003 (DOI: will be inserted by hand later) The Everchanging Pulsating White Dwarf GD358 S.O. Kepler1, R. Edward Nather2, Don E. Winget2, Atsuko Nitta3, S. J. Kleinman3, Travis Metcalfe2;4, Kazuhiro Sekiguchi5, Jiang Xiaojun6, Denis Sullivan7, Tiri Sullivan7, Rimvydas Janulis8, Edmund Meistas8, Romualdas Kalytis8, Jurek Krzesinski9, Waldemar OgÃloza9, Staszek Zola10, Darragh O’Donoghue11, Encarni Romero-Colmenero11, Peter Martinez11, Stefan Dreizler12, Jochen Deetjen12, Thorsten Nagel12, Sonja L. Schuh12, Gerard Vauclair13, Fu Jian Ning13, Michel Chevreton14, Jan-Erik Solheim15, Jose M. Gonzalez Perez15, Frank Johannessen15, Antonio Kanaan16, Jos´eEduardo Costa1, Alex Fabiano Murillo Costa1, Matt A. Wood17, Nicole Silvestri17, T.J. Ahrens17, Aaron Kyle Jones18;¤, Ansley E. Collins19;¤, Martha Boyer20;¤, J. S. Shaw21, Anjum Mukadam2, Eric W. Klumpe22, Jesse Larrison22, Steve Kawaler23, Reed Riddle23, Ana Ulla24, and Paul Bradley25 1 Instituto de F´ısicada UFRGS, Porto Alegre, RS - Brazil e-mail: [email protected] 2 Department of Astronomy & McDonald Observatory, University of Texas, Austin, TX 78712, USA 3 Sloan Digital Sky Survey, Apache Pt. Observatory, P.O. Box 59, Sunspot, NM 88349, USA 4 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 USA e-mail: [email protected] 5 Subaru National Astronomical Observatory of Japan e-mail: [email protected] 6 Beijing Astronomical Observatory, Academy of Sciences, Beijing 100080, China e-mail: [email protected] 7 University of Victoria, Wellington, New Zealand 8 Institute of Theoretical Physics and Astronomy, Gostauto 12, Vilnius 2600, Lithuania 9 Mt. Suhora Observatory, Cracow Pedagogical University, Ul. Podchorazych 2, 30-084 Cracow, Poland 10 Jagiellonian University, Krakow, Poland e-mail: [email protected] 11 South African Astronomical Observatory 12 Universitat T¨ubingen,Germany 13 Universit´ePaul Sabatier, Observatoire Midi-Pyr´en´ees,CNRS/UMR5572, 14 av. E. Belin, 31400 Toulouse, France 14 Observatoire de Paris-Meudon, DAEC, 92195 Meudon, France e-mail: [email protected] 15 Institutt for fysikk, 9037 Tromso, Norway 16 Departamento de F´ısica, Universidade Federal de Santa Catarina, CP 476, CEP 88040-900, Florian´opolis, Brazil, e-mail: [email protected] 17 Dept. of Physics and Space Sciences & The SARA Observatory, Florida Institute of Technology, Melbourne, FL 32901 ? 18 University of Florida, 202 Nuclear Sciences Center Gainesville, FL 32611-8300 19 Johnson Space Center, 2101 NASA Road 1, Mail Code GT2, Houston, TX 77058, USA 20 University of Minnesota, Department of Physics & Astronomy, 116 Church St. S.E., Minneapolis, MN 55455 21 University of Georgia at Athens, Department of Physics and Astronomy, Athens, GA 30602-2451, USA 22 Middle Tennessee State University, Department of Physics and Astronomy Murfreesboro, TN 37132, USA 23 Department of Physics and Astronomy, Iowa State University, Ames, IA 50011, USA 24 Universidade de Vigo, Depto. de Fisica Aplicada, Facultade de Ciencias, Campus Marcosende-Lagoas, 36200 Vigo (Pontevedra), Spain e-mail: [email protected] 25 Los Alamos National Laboratory, X-2, MS T-085 Los Alamos, NM 87545, USA Received 6 Dec 2002 / Accepted 22 Jan 2003 Abstract. We report 323 hours of nearly uninterrupted time series photometric observations of the DBV star GD 358 acquired with the Whole Earth Telescope (WET) during May 23rd to June 8th, 2000. We acquired more than 232 000 independent measurements. We also report on 48 hours of time-series photometric observations in Aug 1996. We detected the non-radial g-modes consistent with degree ` = 1 and radial order 8 to 20 and their linear combinations up to 6th order. We also detect, for the first time, a high amplitude ` = 2 mode, with a period of 796 s. In the 2000 WET data, the largest amplitude modes are similar to those detected with the WET observations of 1990 and 1994, but the highest combination order previously detected was 4th order. At one point 2 Kepler et al.: The Everchanging Pulsating White Dwarf GD358 during the 1996 observations, most of the pulsation energy was transferred into the radial order k = 8 mode, which displayed a sinusoidal pulse shape in spite of the large amplitude. The multiplet structure of the individual modes changes from year to year, and during the 2000 observations only the k = 9 mode displays clear normal triplet structure. Even though the pulsation amplitudes change on timescales of days and years, the eigenfrequencies remain essentially the same, showing the stellar structure is not changing on any dynamical timescale. Key words. (Stars:) white dwarfs, Stars: variables: general, Stars: oscillations, Stars: individual: GD 358, Stars: evo- lution 1. Introduction GD 358, also called V777 Herculis, is the prototype of the DBV class of white dwarf pulsators. It was the first pulsating star detected based on a theoretical prediction (Winget et al. 1985), and is the pulsating star with the largest number of periodicities detected after the Sun. Detecting as many modes as possible is important, as each periodicity detected yields an independent constraint on the star’s structure. The study of pulsating white dwarf stars has allowed us to measure the stellar mass and composition layers, to probe the physics at high densities, including crystallization, and has provided a chronometer to measure the age of the oldest stars and consequently, the age of the Galaxy. Robinson, Kepler & Nather (1982) and Kepler (1984) demonstrated that the variable white dwarf stars pulsate in non-radial gravity modes. Beauchamp et al. (1999) studied the spectra of the pulsating DBs to determine their instability strip at 22 400 · Teff · 27 800 K, and found Teff = 24 900 K, log g = 7:91 for the brightest DBV, GD 358 (V=13.85), assuming no photospheric H, as confirmed by Provencal et al. (2000). Provencal et al. studied the HST and EUVE spectra, deriving Teff = 27 000 § 1 000 K, finding traces of carbon in the atmosphere [log(C=He) = ¡5:9 § 0:3] and a broadening corresponding to v sin i = 60§6 km/s. They also detected Ly® that is probably interstellar. Althaus & Benvenuto (1997) demonstrated that the Canuto, Goldman, & Mazzitelli (1996, hereafter CGM) self consistent theory of turbulent convection is consistent with the Teff ' 27 000 K determination, as GD 358 defines the blue edge of the DBV instability strip. Shipman et al. (2002) extended the blue edge of the DBV instability strip by finding that the even hotter star PG0112+104 is a pulsator. Winget et al. (1994) reported on the analysis of 154 hours of nearly continuous time series photometry on GD 358, obtained during the Whole Earth Telescope (WET) run of May 1990. The Fourier temporal spectrum of the light curve is dominated by periodicities in the range 1000 – 2400 ¹Hz, with more than 180 significant peaks. They identify all of the triplet frequencies as having degree ` = 1 and, from the details of their triplet (k) spacings, from which Bradley & Winget (1994) derived the total stellar mass as 0:61 § 0:03 M¯, the mass of the outer helium envelope as ¡6 2:0 § 1:0 £ 10 M¤, the luminosity as 0:050 § 0:012 L¯ and, deriving a temperature and bolometric correction, the distance as 42 § 3 pc. Winget et al. (1994) found changes in the m spacings among the triplet modes, and by assuming the rotational splitting coefficient C`;k(r) depends only on radial overtone k and the rotation angular velocity Ω(r), interpret the observed spacing as strong evidence of radial differential rotation, with the outer envelope rotating some 1:8 times faster than the core. However, Kawaler, Sekii, & Gough (1999) find that the core rotates faster than the envelope when they perform rotational splitting inversions of the observational data. The apparently contradictory result is due to the presence of mode trapping and the behavior of the rotational splitting kernel in the core of the model. Winget et al. also found significant power at the sums and differences of the dominant frequencies, indicating that non–linear processes are significant, but with a richness and complexity that rules out resonant mode coupling as a major cause. We show that in the WET data set reported here (acquired in 2000), only 12 of the periodicities can be identified as independent g-mode pulsations, probably all different radial overtones (k) with same spherical degree ` = 1, plus the azimuthal m components for k = 8 and 9. The high amplitude with a period of 796 s is identified as an ` = 2 mode; it was not present in the previous data sets. Most, if not all, of the remaining periodicities are linear combination peaks of these pulsations. Considering there are many more observed combination frequencies than available eigenmodes, we interpret the linear combination peaks as caused by non-linear effects, not real pulsations. This interpretation is consistent with the proposal by Brickhill (1992) and Wu (2001) that the combination frequencies appear by the non-linear response of the medium. Recently, van Kerkwijk et al. (2000) and Clemens et al. (2000) show that most linear combination peaks for the DAV G29-38 do not show any velocity variations, while the eigenmodes do. However, Thompson et al. (2003) argue that some combination peaks do show velocity variations. As a clear demonstration of the power of asteroseismology, Metcalfe, Winget, & Charbonneau (2001) and Metcalfe, Salaris, & Winget (2002) used GD 358 observed periods from Winget et al. (1994) and a genetic algorithm to search for the optimum theoretical model with static diffusion envelopes, and constrained the 12C(®; γ)16O cross section, a crucial Send offprint requests to: S.O. Kepler, e-mail: [email protected] ? Southeastern Association for Research in Astronomy (SARA) NSF-REU Student. Kepler et al.: The Everchanging Pulsating White Dwarf GD358 3 parameter for many fields in astrophysics and difficult to constrain in terrestrial laboratories.
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